The invention is in the field of electronic reproduction technology and is directed to a method for signal processing in an electronic engraving machine for engraving printing forms, particularly printing cylinders, for rotogravure.
In an electronic engraving machine, for example, an electromagnetic engraving element having an engraving stylus as a cutting tool moves in an axial direction along a rotating printing cylinder. The engraving stylus controlled by an engraving control signal cuts a sequence of depressions, called cups, arranged in a rotogravure raster into the generated surface of the printing cylinder. The engraving control signal is formed in a signal editing unit by superimposition of an engraving signal representing the gradations between xe2x80x9clightxe2x80x9d (white) and xe2x80x9cdarkxe2x80x9d (black) with a periodic raster signal (vibration). Whereas the raster signal effects a vibrating lifting motion of the engraving stylus for generating the raster, the engraving signal controls the cut depths of the cups engraved into the generated surface of the printing cylinder according to the gradations to be reproduced.
DE-A 23 36 089 discloses an electromagnetic engraving element, i.e. an engraving element having an electromagnetic drive element for the engraving stylus. The electromagnetic drive element is composed of a stationary electromagnet charged with the engraving signal in whose air gap the armature of a rotatory system moves. The rotatory system is composed of a shaft, the armature, a bearing for the shaft and of a damping unit. One shaft end merges into a resilient torsion rod clamped stationary in space, whereas the other shaft end carries a lever to which the engraving stylus is attached. The magnetic field generated in the electromagnet exerts an electrical torque onto the armature of the shaft, this being opposed by the mechanical torque of the torsion rod. The electrical torque deflects the shaft out of a quiescent position by a rotational angle proportional to the engraving signal, and the torsion rod returns the shaft into the quiescent position. As a result of the rotational movement of the shaft, the engraving stylus implements a stroke directed in the direction onto the generated surface of the printing cylinder, this defining the penetration depth of the engraving stylus into the printing cylinder.
Since the electromagnetic engraving element represents an oscillatable system, the engraving stylus exhibits a faulty trenchant response, particularly given engraving signal discontinuities at steep contours, that considerably deteriorates the engraving quality.
In what is referred to as the lag effect, the engraving stylus achieves the rated engraving depth at a contour predetermined by the engraving signal value only with a delay, and an unsharp engraving of an intrinsically steep contour is the result. The cause of the lag effect is, for example, the non-ideal properties of the mechanical damping.
In what is referred to as the rebound effect, the engraving stylus executes oscillations with the mechanical inherent frequency at a contour due to an inadequate mechanical damping, and the engraving stylus engraves a disturbing multiple contour.
In what is known as the hysteresis effect, the engraving stylus, due to the non-ideal properties of the mechanical damping, never achieves the rated engraving depth prescribed by the engraving signal value. When, for example, a black tone is engraved in a gray area, the hysteresis effect is expressed in that a darker gray arises behind the black gradation.
EP-B-0 437 421 has already disclosed a method with which the trenchant response of an electromagnetic engraving element is improved with a specific electronic drive of the engraving element. For that purpose, the engraving signal is briefly intermediately stored in a memory unit and is supplied to the engraving element delayed by the storage time. During the storage time, a correction signal variable in amplitude and duration of effect is derived from the engraving signal, this being supplied to the engraving element with a time advance. The compensation of the aforementioned, disturbing effects of the engraving element, the generation of the raster signal and the superimposition of the raster signal with the corrected engraving signal in order to acquire the engraving control signal for driving the engraving element are among the things that occur the signal editing unit.
In a traditional signal processing unit, the signals are processed analog, in that the input signals are supplied in digital form, are digital-to-analog converted and are operated with one another in analog networks, whereby the resulting signal is then amplified in an analog amplifier and is output as engraving control signal for the engraving element.
The compensation of the disturbing effects of the engraving element with analog networks has the disadvantage that the transfer behavior of the analog networks cannot be optimally adapted without further ado to the transfer behavior of the engraving elements, and that the analog networks, due to tolerances and temperature-dependencies of the components, are not stable enough in order to generate a good long-term stability and, thus, engraving quality.
WO-A-9 634 746 discloses a method for signal processing in an electronic engraving machine, whereby the engraving data are already subjected to a digital filtering. The disadvantage of this method is comprised therein that the digital filtering ensues after the superimposition of engraving signal and raster signal.
It is therefore an object of the present invention to improve a method for signal processing in an electronic engraving machine for engraving printing forms, particularly printing cylinders, for rotogravure such that disturbing effects of an engraving element are compensated as completely as possible in order to achieve a good engraving quality.
According to the present method for signal processing in an electronic engraving machine for engraving a printing form for rotogravure, engraving data are provided to represent gradations between xe2x80x9cwhite and xe2x80x9cblackxe2x80x9d to be engraved. The engraving data are subjected to a digital filtering for compensation of a faulty lifting motion of an engraving stylus of an engraving element. An engraving control signal is acquired by superimposition of the engraving data with a periodic raster signal for generating an engraving raster. The engraving control signal controls a lifting motion of the engraving stylus of the engraving element. A sequence of cups arranged in the engraving raster is engraved into the printing form with a lifting motion of the engraving stylus.
The invention is explained in greater detail below with reference to FIGS. 1 through 9.